1. Field of the Invention
The present invention relates to electrostatic image developing toner for visualizing an electrostatic latent image formed in an electrophotographic method, an electrostatic printing method, an electrostatic recording method, or the like, and an image forming method using the electrostatic image developing toner.
2. Background Art
Of the printing or recording methods, for example, in the electrophotographic method, a photoconductor is charged and exposed so that an electrostatic latent image is formed on the photoconductor. Next, this electrostatic latent image is developed with fine-grained toner containing a coloring agent and so on, using resin as a binder. The obtained toner image is transferred and fixed onto recording paper to obtain a recorded image.
In such an electrostatic image recording process, the step of developing an electrostatic latent image with fine-grained toner, and the step of fixing a toner image onto recording paper are particularly important. In the related art, a magnetic brush developing method using a two-component developer composed of toner and magnetic carrier capable of performing high-speed and high-quality development is often used as a method for developing an image with toner. In addition, a heat roller fixing method which is high in thermal efficiency and capable of fixing toner at a high speed is often used as a method for fixing the toner.
On the other hand, recently, with the development of an information apparatus, laser beam printers have made an advance. In such a laser beam printer, a laser beam is used for exposing a photoconductor so as to reproduce every dot for a recorded image by a modulating signal based on instructions from a computer. Particularly, in a recent laser beam printer, the diameter of a laser beam is narrowed down to increase the dot density to 600-1,200 dpi (dots/inch) because the laser beam printer is requested to produce a higher quality image. With the increase of the dot density, the particle sizes of toner and carrier become smaller to develop a fine electrostatic latent image. Thus, application of fine-grained toner whose volume average particle size is not larger than 10 xcexcm, and fine-grained carrier whose weight average particle size is not larger than 100 xcexcm is advancing.
On the other hand, heat roller fixing as described above is often used for the fixing. However, development of toner which can be fixed with reduced power consumption of a fixing heater and a driving motor, and with lower temperature and lower pressure of a heat roller has been desired from the following points of view. That is,
(1) To restrain the printer from being deteriorated due to overheating, and to prevent parts in the printer from being thermally deteriorated;
(2) To shorten warm-up time from the time when a fixing unit is actuated to the time when fixing becomes possible;
(3) To prevent a failure in fixing due to heat absorption into recording paper, so as to make it possible to keep image quality while feeding the paper continuously;
(4) To prevent the recording paper from being curled and fired due to overheating; and
(5) To reduce a load on the heat roller, and to simplify and miniaturize the structure of the fixing unit.
In this manner, development of high-performance toner which is of fine particles and which can be fixed at low temperature and with low pressure has been desired. On the other hand, when the toner is formed into fine particles not larger than 10 xcexcm as described above, there arise problems as follows. That is, fine-grained toner used in the developing step indeed brings about high image quantity, but easily causes toner adhesion (fogging) to a non-image area, and toner flying thereon. Accordingly, the handling properties in toner shipping or the like are also easily degraded due to the lowering of fluidity.
Further, due to the strength of adhesion and the weakness of crashworthiness of the fine-grained toner, carrier pollution (carrier spent) with the toner is easily produced so that the life of developer is easily reduced. In addition, in order to obtain the same fixing strength for the fixing, more energy is required than in the case where toner larger in particle size is used. Further, the yield in the steps of graining and classifying in manufacturing the toner is reduced so that the cost of the toner increases.
Fine-grained toner brings about such a large number of problems. It is usually difficult to put toner smaller than 5 xcexcm in average particle size into practical use. Therefore, toner classified to have an average particle size in a range of from 5 xcexcm to 10 xcexcm is used with the fluidity of the toner being enhanced by the improvement of external additives to the toner and the recipe for the external additives. On the other hand, the weight average particle size of the carrier is set to be not larger than 100 xcexcm with the reduction in particle size of the toner. Thus, the specific surface area of the carrier is increased to improve the frictional electrostatic property with the toner. However, when the carrier is smaller than 30 xcexcm, the magnetic force of the carrier is reduced to easily adhere onto an electrostatic image holding member due to electrostatic attraction force. Therefore, carrier classified to have an average particle size in a range of from 30 xcexcm to 100 xcexcm is used, and the surface of the carrier is coated with resin in accordance with necessity.
As a result of these improvements in the particle size distribution and in the fluidity and the electrostatic property, fine-grained toner and developer have been put into practical use in copying machines, printers, etc. However, when printing is performed with an actual apparatus, particularly when printing at a high speed not lower than 10 pages per minute is repeated, the fine-grained toner has its own peculiar problems as described above. Thus, the life of developer is reduced easily due to carrier spent by the toner, and the life of a photoconductor is reduced easily because the photoconductor is filmed with the toner.
In addition, it is difficult to obtain fixing strength of an image. Particularly in the fixing step, it is necessary to increase the temperature and pressure of a heat roller. Thus, there has been a problem that it is difficult to make a fixing unit highly reliable, simple, small in size and low in cost.
In order to improve the fixing performance of toner, it has been known to add wax to fixing resin. For example, such techniques are disclosed in Japanese Patent Laid-Open No. 3304/1977, No. 3305/1977 and No. 52574/1982.
Waxes are used to prevent toner from adhering to a heat roller at a low temperature or at a high temperature, that is, to prevent a so-called offset phenomenon, so as to improve the toner fixability at a low temperature. Recently, low-melting waxes attract attention from the point of view of low-temperature fixation.
For example, Japanese Patent Laid-Open No. 313413/1993 discloses that ethylene- or propylene-xcex1-olefin copolymer whose viscosity is not higher than 10,000 poises at 140xc2x0 C. is added to vinyl-based copolymer having a particular molecular weight distribution in order to improve the low-temperature fixability, the offset resistance and the non-aggregability of toner.
In addition, for the similar purpose, Japanese Patent Laid-Open No. 287413/1995 discloses that paraffin wax showing a peak (melting point) at 75-85xc2x0 C. in the amount of heat absorption measured by a differential scanning calorimeter (DSC) is added; Japanese Patent Laid-Open No. 314181/1996, No. 179335/1997 and No. 319139/1997 disclose that natural-gas-based Fischer-Tropsch wax whose melting point measured by a DSC is 85-100xc2x0 C. is added; Japanese Patent Laid-Open No. 324513/1994 discloses that polyethylene wax whose melting point measured by a DSC is 85-110xc2x0 C. is added; Japanese Patent Laid-Open No. 36218/1995 discloses that polyethylene-based wax in which any component having a melting point at 50xc2x0 C. or lower has been eliminated in a distillation method and whose melting point measured by a DSC is 70-120xc2x0 C. is added; and Japanese Patent Laid-Open No. 114942/1996 discloses that polyethylene wax whose weight average molecular weight (Mw) is lower than 1,000 is added.
On the other hand, when low-melting wax is added to toner, the toner deteriorates in heat resistance, durability, storage stability, and fluidity. In order to improve those properties, Japanese Patent Laid-Open No. 123994/1994 discloses that wax not higher than 1.5 in ratio (Mw/Mn) of weight average molecular weight to number average molecular weight is used; Japanese Patent Laid-Open No. 209909/1995 discloses that ethylene-based olefin copolymer wax whose melt viscosity is 0.5-10 mPaxc2x7s at 140xc2x0 C. and whose penetration degree is not higher than 3.0 dmm is used; and Japanese Patent Laid-Open No. 287418/1995 discloses that Fischer-Tropsch wax whose average molecular weight is not lower than 1,000 is used.
The fixing performance of toner can be indeed improved using such related-art techniques. When low-melting wax is used, particularly when toner is of fine particles, it is however difficult to improve the fixing performance while keeping the heat resistance, the durability, the storage stability and the fluidity of the toner. Thus, it is not possible to provide toner capable of being put into practical use, and an image forming method using the toner.
An object of the invention is to provide toner in which the heat resistance, the durability, the storage stability and the fluidity of the toner are so excellent that the life of developer is hardly reduced due to carrier spent by the toner, and the life of a photoconductor is hardly reduced due to filming of the photoconductor with the toner; in which energy required for fixing is so small that temperature and pressure of a heat roller can be reduced when a heat roller fixing method is adopted; and in which an offset phenomenon is hardly produced. Another object of the invention is to provide a stable image forming method using such toner.
The object is achieved by electrostatic image developing toner including at least fixing resin and wax, wherein the wax is hydrocarbon-based wax, containing, as its constitutional components, first wax and second wax; the first wax is higher than 1.5 in ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), lower than 10 mPaxc2x7s in melt viscosity at 140xc2x0 C. and higher than 75% and not higher than 85% in degree of crystallinity; and the second wax is not higher than 1.5 in ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), lower than 10 mPaxc2x7s in melt viscosity at 140xc2x0 C. and higher than 85% and not higher than 95% in degree of crystallinity.